The present invention is applicable to silicon carbide and carbon surfaces of all shapes and sizes. It is particularly important for filaments, thin strips, and the like. The following discussion will be directed to filaments as a typical example.
1. A silicon carbide (SiC) monofilament is a monofilament that derives its properties from a bulk layer of essentially stoichiometric SiC.
2. An outer layer or surface coating is a layer of material deposited on a SiC monofilament. A surface layer may be deposited directly on the bulk layer or on an intermediate layer of material positioned between the bulk layer and the exterior layer.
3. For purposes of this invention, the term "carbon-rich SiC layer" shall apply to a carbonaceous SiC containing layer or surface on a SiC monofilament where the ratio of silicon to carbon is less than stoichiometric but may contain up to 75% carbon. Previous coatings (for example, U.S. Pat. No. 4,340,636) contained up to 100% carbon.
Composite materials in plastic or metal matrices reinforced with high-strength, high-modulus filaments such as boron and silicon carbide are finding increased popularity in structural applications. In particular these types of composites are useful where high-strength and stiffness with accompanying low weight is desired.
The field of technology defined as high-strength, high-modulus monofilaments is unique and ultracritical to changes in structure or process of manufacture.
Persons skilled in the art of high-strength, high-modulus filaments have observed that it is frequently not possible to predict what effect changes in compositions, processes, feedstocks, or post-treatments will have on the properties of this unique family of filaments.
Boron nitride, boron carbide, titanium nitride, titanium carbide, and tungsten in combination with boron or silicon carbide monofilaments in specific applications have failed to provide a useful monofilament, though in each instance, the candidate material was chosen to enhance one or more properties of the high-strength monofilament.
One form of a silicon carbide surface layer on a silicon carbide monofilament would not protect the filament from degradation unless the coating had a critical cross-section profile. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Carbon cores heretofore required buffer layers deposited at critical specific temperatures. See U.S. Pat. No. 4,142,008.
Forms of carbon-rich silicon carbide surface layers on SiC monofilaments which exhibit high tensile strengths were found wanting in transverse strength for example, when the monofilaments were used to form metal matrix and resin matrix composite materials. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Other structural or manufacturing procedural features which were found to be of a critical nature are change in core composition and surface texture, the presence or absence of a buffer layer of specific compositions, impurities, and reactivity of a surface coating with matrix material.